Since Unicode 2.0, the authors of the Unicode Standard noticed that the original 65,536 characters are not adequate for the modern needs of information processing (Han characters were proving to be too many, if one considered the rarely-used characters).

To handle this, they extended the upper limit of Unicode from U+FFFF to U+10FFFF, thereby introducing more than a million new character slots. The authors of the Unicode Standard, guessing many people will take a little time to change the architecture of their software to support the full repertoire of Unicode, only encoded characters of less frequent use (like rarely-used Han characters, older scripts, or musical notation) in that area.

The range U+0000 to U+FFFF, where more important characters are stored, is called the Basic Multilingual Plane, while the range U+10000 to U+10FFFF is divided into 16 planes, only three of which have so far been used to encode characters.